It should be pretty simple; note that the name attribute on the select is dispmax. So that's the value you need to send to the CGI that handles the form submission. Submitted variables, when using a GET request (as you are here) are separated by ampersands and take the form name=value (if that's confusing you, just look a the URLs you are already using for an example), so if you simply add &dispmax=500 to the URL that you are fetching, that ought to do it.

P.S. This is more of an HTML question than a Perl question. You might want to try W3C Schools to learn more about HTML.